1. Field of the Invention
The invention relates to automatic testing and, more particularly, to testing a device with a programmed digital computer.
2. Description of the Prior Art
Intermittent failures in complex digital electronic devices, such as microprocessor-controlled copiers, are extremely difficult to identify and correct. Because the failures occur infrequently, repair personnel must watch large numbers of points for long periods of time merely to observe a single failure; correction of the cause requires repeated observations. Standard cathode ray oscilloscopes transiently monitor only a few points and do not record observed phenomena for later analysis. Chart recorders make a paper record of relatively slow signals not commonly monitored in complex digital devices.
Logic analyzers simplify repair by simultaneously watching many points and displaying signals at these points only when prespecified failures occur. An article in Computer Design, March 1981, "Logic Analyzers Simplify System Integration Tasks" pages 119-126 describes displaying signals, at monitored points of a tested device, only when these signals occur within a specified address range or time interval. U.S. Pat. No. 3,831,149, issued Aug. 20, 1974, describes preselection of conditions under which data received from a tested system will be recorded (viz. identity between preset data and test data either records the test data only or starts recording from that time on). A circuit in U.S. Pat. No. 3,927,310, issued Dec. 16, 1975, strobes a display when preselected timing signals occur. In U.S. Pat. No. 3,976,864, issued Aug. 24, 1976, pulses start and stop data signature generation in accordance with preselected patterns and qualifiers which are compared with data conditions in a device under test. Apparatus described in U.S. Pat. No. 3,818,201, issued June 18, 1974, freezes a display of sampled signals upon detection of predetermined coincidences between sampled signals and external keys.
The prior art does not show testing apparatus which repeatedly displays portions of signals, at a plurality of test points in a tested digital device selected in accordance with predefined conditions and signals occurring at the test points.